Once it is in orbit and it has delivered its payload, the only place for it to go is back to earth.
"This opens new frontiers for exploration of the Solar System as the rocket can be refueled in-orbit and re-utilize its aerospike engine thus eliminating the need for additional upper stages."
They eschew the need for extra stages and then decide that an additional vehicle is necessary for refueling. What good is that if no refueling vehicle is available. Also not very good for higher orbits or polar orbits.
Once it is in orbit and has delivered its payload, it will be in orbit, and not fall back to earth. This is what they were talking about with the refueling. Once they have some rockets hanging out in space, they can bring up fuel and get it to travel further out. You can go a lot farther on a tank when you start from orbit rather than on the ground, and the engine makes more power to boot.